To reach classically trained musicians, the letters are good but I think notation is BEST. Being a music reader I can look at a score and get a sense of it right away, and no theory is needed - not all classical performers are good at theory despite perhaps being forced to study it. I remember being turned down for High School Jazz band because I didn't know chord symbols, despite years of piano lessons. Once I learned how to read/make my own chord charts a whole world opened up to me...
The other thing is RHYTHM. Many Classical players are weak in this area, and Western notation can't capture all the nuances of Jazz or Gospel performance. But try - include some examples of rhythm patterns. IMO, many Jazz books get too heavy on the theory and don't put enough emphasis on rhythm and soloing/comping balance with other players.
Of course there is no substitute for playing/listening to good Gospel players...
